I. Field of the Invention
This invention relates to the formation of coated polymer sheets having good oxygen and moisture barrier properties suitable for use as packaging materials for foodstuffs and the like. More particularly, the invention relates to a process for producing such materials involving the application, of barrier layers to polymer sheets preferably made of transparent plastic. The invention also relates to the coated sheets thus formed and to apparatus used for the process.
II. Description of the Prior Art
Plastic packaging sheets used in the food industry are normally made moisture and oxygen impermeable by coating the plastic sheets on one side with a relatively thick layer of aluminum. The resulting sheet is opaque, so that food contents cannot be seen, and the sheets cannot be used in microwave ovens because of undesirable shorting and reflections caused by the metal layer.
There is a need for transparent, microwavable packaging sheets having the required barrier properties. While multi-layer plastic laminates can be used to reduce the oxygen and water vapour transmission characteristics of packaging materials, satisfactory structures are very expensive and often require as many as six different polymer layers (see Modern Plastics, August 1986, pp 54-56).
In recent years, a different approach to the problem has consisted of vacuum depositing thin films of inorganic coatings onto flexible transparent polymer laminates (see, for example, U.S. Pat. No. 4,702,963 issued on Oct. 27, 1987 to Optical Coating Laboratory Inc. and Japanese Patent Application 60 46,363). A recent article in Paper, Film and Foil Converter, June 1988, pp 102-104, describes the deposition of transparent silica barrier coatings on plastic films via electron beam technology. It is apparent that complex and expensive equipment has to be utilized to deposit such barrier coatings onto plastic substrates and that the resulting coatings may be subject to cracking upon flexing of the film. Furthermore, the silica type films used in the process exhibit a yellowish discolouration when laminated with transparent flexible polymer films for use in packaging, and this discolouration makes many food contents look unappealing. Finally, materials deposited by electron beam techniques are typically less dense than the bulk form of the coating material and so the barrier properties are not optimal.
Non-porous oxide films produced on certain valve metals by anodization are denser than similar materials deposited by electron beam techniques or other types of deposition. However, such films cannot be easily separated over large areas from the metal on which they are formed. Dissolving away the underlying metal base by chemical means would be a possible approach, but would be highly uneconomical and cumbersome and would be difficult to achieve without the oxides themselves being subject to inadvertent dissolution by such means.
An object of the invention is therefore to provide a process for producing a coated polymer sheet having good oxygen and moisture characteristics.
Another object of the invention is to provide a packaging material with a transparent dense anodic oxide coating capable of acting as an oxygen and moisture barrier.
Yet another object of the invention is to provide a packing film with a dense coating of a valve metal oxide.